added: pretty-print hessian backend for NonLinMPC

Author: franckgaga

src/controller/nonlinmpc.jl

@@ -981,4 +981,5 @@ end
 function print_backends(io::IO, mpc::NonLinMPC)
     println(io, "├ gradient: $(backend_str(mpc.gradient))")
     println(io, "├ jacobian: $(backend_str(mpc.jacobian))")
+    println(io, "├ hessian: $(backend_str(mpc.hessian))")
 end

src/general.jl

@@ -94,6 +94,7 @@ function diffmat2vec!(v::AbstractVector, A::AbstractMatrix, i_vec::Vector{Tuple{
 end
 
 backend_str(backend::AbstractADType) = string(nameof(typeof(backend)))
+backend_str(backend::Nothing) = "nothing"
 function backend_str(backend::AutoSparse)
     str =   "AutoSparse ($(nameof(typeof(backend.dense_ad))),"*
             " $(nameof(typeof(backend.sparsity_detector))),"*